Lithium-ion electrochemical cells are commonly used to power portable electronic devices such as mobile phones, personal computers, and personal entertainment modules. Lithium-ion cells are currently being developed for use in electric vehicles because they are capable of providing high energy density. Some of the challenges for lithium-ion cell technology are to develop cells that have good thermal stability, cyclability, and can be manufactured at a reasonable cost.
Negative electrodes (anodes) for use in lithium-ion electrochemical cells have been developed that include certain main group metals and alloys such as silicon and tin. Selected main group metals and alloys can reversibly alloy with lithium electrochemically and can provide much higher densities of lithium storage than carbonaceous negative electrode materials, which are typically used as lithium ion negative electrode materials for electronic devices.
Typically, composite electrodes are prepared or formed by dispersing fine powders of these metals or alloys, a conductive diluent, and a polymeric binder in an appropriate solvent such as N-methyl-pyrrolidinone (NMP). The dispersion can then be coated onto a current collector, typically a copper or aluminum metal foil, and then dried at elevated temperature to remove the solvent. Often the resulting composite electrode is compressed mechanically to reduce the pore void volume in the composite and to increase electrical conductivity between the composite and the current collector.
Composites containing silicon or silicon alloys can store large amounts of lithium since silicon is known to reversibly alloy electrochemically with lithium and store in excess of four equivalents of lithium per equivalent of silicon. However, insertion of four equivalents of lithium can also be associated with an approximate volume change of nearly 300%. This dramatic volume change can result in poor reversibility of lithium insertion/de-insertion due to changes in morphology of the composite from expansion and loss of electrical conductivity between the current collector and the composite if the binder cannot accommodate the volumetric changes.
There is a need for electrodes for use in lithium-ion electrochemical cells that have high energy density. There is also a need for electrodes that have high thermal stability and can reversibly store lithium for a large number of charge/discharge cycles. There is also a need for electrodes that are easy to manufacture and relatively low cost.